1. Field of the Invention
This invention relates generally to an improved process and related apparatus for use in filling the pressure reservoir of an implantable medication infusion pump with a selected pressure fluid, and more particularly to an improved pressure reservoir filling process and related process stations for filling the pressure reservoir with a calibrated quantity of relatively purified pressure fluid, and for testing the performance characteristics of the pressure reservoir under simulated implantation conditions.
Medication infusion pumps are generally known in the art for use in delivering a selected medication to a patient in a scheduled or preprogrammed manner. In recent years, such infusion pumps have been developed in compact form adapted for direct implantation into the body of a patient, and to deliver a specific medication such as insulin to the patient in discrete dosages over an extended period of time. An implanted infusion pump of this general type typically includes an internal medication chamber for receiving and storing a supply of the selected medication in liquid form, with the medication being subjected to a predetermined storage pressure to insure accurate and repeatable delivery conditions through the use of a miniature pump and associated programmable control means. In many cases, the storage pressure is desirably less than ambient body pressure to prevent undesired leakage of the medication from the medication chamber into the body of the patient, and to thereby positively prevent accidental overdose during certain failure modes. For one illustrative example of an implanted medication infusion pump of this general type, see U.S. Pat. No. 4,573,994, to Fischell.
In the past, the medication within the pump medication chamber has been subjected to the desired storage pressure by forming at least a portion of the medication chamber as a movable wall shared with an adjacent pressure reservoir charged with a selected pressure fluid. More particularly, the pressure fluid has comprised a selected fluorocarbon or the like which undergoes liquid-vapor change of state at normal body temperature to appropriately expand or contract the pressure reservoir in a manner acting through the movable wall to maintain the medication chamber under substantially constant pressure conditions.
As the medication chamber is filled, the pressure fluid undergoes significant state change to the liquid phase to reduce the volumetric size of the pressure reservoir. Conversely, as the medication is delivered in doses to the patient, the pressure fluid progressively undergoes state change to the vapor phase to maintain the medication under substantially constant pressure. Freon 113 has been used as the pressure fluid to maintain the medication at a slight negative or subambient pressure in response to normal patient body temperature and altitudinal variations up to 8,500 feet above sea level.
While Freon 113 and similar fluorocarbon materials theoretically maintain the medication under controlled pressure conditions for accurate and repeatable administration to the patient, the actual performance of such fluorocarbon materials is easily and significantly altered in the presence of contaminants within the pressure reservoir. More specifically, small quantities of contaminating air and water are readily ingested by the fluorocarbon material, with the result that the contaminated pressure fluid within the pressure reservoir often does not maintain the medication at the predetermined and substantially constant design pressure.
The impact of such contaminants can be especially pronounced when the medication chamber is completely or nearly filled with liquid medication, such that the pressure reservoir has a substantially minimum volumetric size with the pressure fluid in a predominantly liquid phase state. To avoid or minimize the effects of such contaminants, the medication chamber has typically been filled to a level significantly below the maximum chamber volume or capacity. Unfortunately, this approach fails to optimize the medication-containing capacity of the implantable infusion pump, resulting in a requirement for refilling of the medication chamber at shorter intervals. Alternatively, comparatively larger infusion pumps having larger capacity medication chambers have been needed to increase the medication-containing capacity of the implantable pump.
Too much fluid in the pressure chamber can cause problems with nucleation of the fluid when the reservoir is fully filled to capacity, reducing the volume of the pressure chamber to its minimum size. On the other hand, too little fluid in the pressure reservoir will not provide sufficient pressure (be it either positive or negative) on the medication chamber wall shared in common with the pressure chamber. Thus, it would be highly desirable to somehow calibrate the amount of fluid sealed in the pressure reservoir.
There exists, therefore, a significant need for improved processes and related apparatus for filling the pressure reservoir of an implantable medication infusion pump with a specific or calibrated quantity of a selected and substantially contaminant-free pressure fluid. There exists further a need for a method and related apparatus for verifying proper operation of the pressure fluid during simulated implantation conditions to maintain liquid medication under substantially constant pressure, prior to implantation surgery. The present invention fulfills these needs and provides further related advantages.